Quora, the knowledge-sharing social networking site, announced on Monday a data breach affecting about 100 million of its users. The company discovered the breach on Friday, and a breach investigation is ongoing.

The company’s Chief Executive Officer, Adam D’Angelo, wrote in a blog post that the following data elements were compromised or stolen:

"a) Account information, e.g. name, email address, encrypted password (hashed using bcrypt with a salt that varies for each user), data imported from linked networks when authorized by users; b) Public content and actions, e.g. questions, answers, comments, upvotes; and c) Non-public content and actions, e.g. answer requests, downvotes, direct messages (note that a low percentage of Quora users have sent or received such messages)"

Quora has cancelled affected users' passwords. Quora does not yet know exactly how unauthorized persons accessed its system. The breach announcement did not state when the intrusion began. D'Angelo added:

"We're still investigating the precise causes and in addition to the work being conducted by our internal security teams, we have retained a leading digital forensics and security firm to assist us. We have also notified law enforcement officials."

Affected users are being notified via email. Affected users returning to the site must reset their accounts with new passwords. Quora encourages users with questions to visit its breach help site. Users are warned to change their online passwords.

"... the incident was unlikely to result in identity theft, as the site does not collect sensitive information such as credit card or Social Security numbers... 300 million people around the world use its site at least once a month to ask and answer questions about politics, faith, calculus, unrequited love, the meaning of life and more. By comparison, Twitter claims 326 million monthly active users. But since it blasted onto the social media landscape in 2010, igniting a blaze of interest among tech company employees, Quora has not become the mainstream cultural force that Twitter has..."

This breach is another reminder to all consumers to never use the same password at multiple sites. Cybercriminals are persistent, and will reuse stolen passwords to see which other sites they can break into to steal sensitive personal and payment information.

If you received an email breach notice from Quora, please share it below (after deleting any sensitive personal data).